1. Field of the Invention
The invention is generally related to the field of wet scrubbing for flue gas desulfurization (FGD) and more particularly to an air sparger assembly for an agitation tank.
2. General Background
Utility and industrial combustion systems, such as power plant boiler combustion chambers, that use high sulfur fuels tend to release sulfur-containing compounds such as sulfur dioxide and sulfur trioxide into the flue gases. Due to the detrimental effects of such compounds, it is necessary to prevent their release into the surrounding environment.
A typical FGD wet scrubber has several spray levels that introduce a slurry with pulverized limestone through spray nozzles. The slurry spray removes the sulfur compounds from the flowing flue gas, forming calcium sulfite. The slurry spray droplets eventually fall onto a liquid level in the lower section of the wet scrubber, which is a tank for recirculating the slurry to the various spray levels. The most economical wet scrubbing process for most electric utility power plants is a limestone forced oxidation chemical process. This process injects air through spargers in the recirculation tank to oxidize the calcium sulfite to calcium sulfate (gypsum). The chemically stable calcium sulfate solids are filtered and dried from a slurry blowdown stream. Also within the tank are several shaft-driven impellers or mixers that agitate the tank to promote mixing and to prevent solids from settling to the bottom of the recirculation tank.
Current oxidation air spargers are designed to uniformly distribute air across the tank. The current air sparger design is characterized by several parallel pipes at a common elevation above the tank floor. There is a minimum height requirement of five to ten feet between the sparger pipe and the mixers in order to avoid mechanical and functional interaction between the two components. This height is dependent on the horsepower of the mixers and increases as the mixer horsepower increases. Each pipe has holes drilled in the lower section of the pipe to discharge air downward. The holes are typically sized to obtain about a one psi air pressure drop at the design air flow. The air flow rate is established by the chemical process objectives and the mass transfer efficiency. At the end of each pipe is a dip leg that is partially filled with slurry to balance the air pressure within the sparger pipe. The parallel sparger pipes are designed to uniformly distribute the air flow across the tank area. The main air flow header that delivers the air from the compressors is typically perpendicular to the parallel sparger pipes within the tank, a common practice for retrofit installations. Alternately, the main air flow header can be located outside the tank with the individual sparger pipes penetrating the recirculation tank, a common practice for new installations. The uniform distribution of air flow inhibits tank mixing by forming many small vortices above each individual sparger pipe. These small vortices are formed by the bubbles moving upward and liquid flow between the spargers moving down. Since the sparger pipes are closely spaced, the downward flow does not penetrate very far below the sparger pipe level. Thus, the sparger pipes tend to isolate the tank into an upper and lower section. The mixer impellers thus must be located below the sparger pipes. An alternate approach for oxidizing and mixing the recirculation tank is to inject the oxidation air via a pipe in the immediate vicinity of a mixer impeller. The impeller and flow of slurry breaks up the air jet into smaller bubbles throughout the tank. Another disadvantage in such systems is that the loss of a mixer affects the capacity of the system to achieve oxidation unless a spare mixer is provided.